Sample presentation strategies for FT-IR spectroscopy in food applications

A variety of analytical methods is used in both, academia and industry for the analysis of food and beverage products. Standard wet chemistry based methods often suffer from a high instrument cost, laborious and time-consuming sample preparation and the need for skilled operators. Because of these problems with most instrumental techniques nowadays used for food analysis, there is a need for alternative methods and techniques that are fast, simple and reliable. Fourier Transform Infrared (FT-IR) spectrometry is an emerging technique in food science that potentially fulfills these requirements. FT-IR spectrometry is based on the absorption of IR light by the chemical components present in a sample. The IR spectrum is unique for every molecular structure and the intensity of absorption is directly related to the concentration of the absorbing species. FT-IR spectrometry is a very versatile method with respect to sample presentation and includes many different strategies appropriate in each particular application. The knowledge of the optimal FT-IR sample presentation method is important for the development of the methodology and its implementation in practical settings. The main objective of this thesis was to study the potential of different sample presentation strategies of FT-IR spectroscopy in food applications.In the first part of the thesis, two different FT-IR sampling techniques were studied in greater detail with respect to food analysis: the attenuated total reflection accessory (ATR) and a novel high-throughput screening module (HTS). In ATR, the evanescent IR wave is produced by transmitting the IR beam through a crystal of high refractive index. To obtain a signal the generated evanescent wave is brought into contact with a sample. In HTS the IR radiation is transmitted through the microplate with samples on top of it and allows for fast analysis of many samples in short time. The potential of both ATR and HTS strategies of FT-IR spectrometry was evaluated in two food systems beer and milk. An advanced data analysis approach, which included empirical estimation of the best spectral preprocessing and variable selection method, was used for prediction of quality parameters and components of beer and milk samples based on their IR spectra. The results showed that both ATR/FT-IR and HTS/FT-IR can be used as a rapid analytical technique for the determination of major quality parameters and components in food samples, and therefore, has the potential to be implemented in real industrial settings. Even though the prediction results for HTS were found to be worse than for ATR, this method can still be useful for screening major components of large sample numbers of various food products. In the second part of the research, a microfluidic lab-on-a-chip platform was developed as a novel sample presentation strategy. It was specifically designed for compatibility with FT-IR microspectroscopic detection. FT-IR microscopy with a focal plane array (FPA) detector allowed to take chemical snapshots of the sample by acquiring thousands of IR spectra simultaneously, measured from a specific location of the sample. This provided not only spectral, but also spatially resolved information. First, as a proof-of-concept the microfluidic presentation method was used successfully for FT-IR chemical imaging of continuous and segmented flow in the microchannels. Second, a kinetic study of enzymatic reaction of D-glucose with glucose oxidase as a model system was carried out and compared to the ATR/FT-IR measurements. The obtained results showed that the integration of microfluidics and FT-IR microscopy could serve as an innovative bio(molecular) detection concept that can be applied for investigation of the kinetics of different biochemical reactions. It offers new opportunities in bioassays and microfluidics.status: publishe